This invention relates generally to power supply voltage regulating circuits and specifically to regulating circuits used in connection with switch mode type power supplies.
It is well known in the art to compensate for voltage variations due to changes in current in a load supplied from the secondary winding of a transformer. This is generally accomplished by adjusting the voltage developed across the secondary winding to increase with an increase in load current and to decrease with a decrease in load current. So-called switch mode power supplies have recently become popular due to their favorable cost and operating efficiency. In some switch mode supplies, the current in a transformer primary winding is switched at a frequency ranging from 20 KHz to 70 KHz by a transistorized switching circuit to develop the secondary voltages.
Feedback regulation, that is, regulation that controls the voltage developed by the primary winding to compensate for changes in circuit load current, "bypasses" the transformer and poses safety problems. Circuits that employ primary winding regulation generally include an opto-coupler to avoid the possibility of the secondary winding of the transformer being exposed to primary winding voltage. Opto-couplers, however, not only have slow response times, but are relatively expensive. Also, regulation of the voltage across the primary winding of the power supply transformer changes the voltage across the entire secondary winding. Since it is common to have a number of different circuits independently supplied from the secondary winding, an increase in loading of one circuit will be compensated by changing a voltage that is applied to all circuits. Thus, a nominally operating circuit will be adversely affected by compensating for a circuit that experiences an increase or a decrease in loading. Typically, one or more 12 volt (12 V) load circuits may be supplied by a secondary winding along with a 5 volt (5 V) logic voltage load circuit. While the voltages to the 12 V load circuits may require some regulation, the 5 V logic voltage load circuit needs very close regulation. The 5 V logic voltage load circuit is also generally subject to a significant amount of load current variation. Consequently, if the 5 V supply to the logic load is controlled or regulated with reference to the voltage of the primary winding of the transformer, the 12 V load circuits will experience undesirable changes in operating potential. The converse is, of course, also true.
A different, though somewhat related, problem exists with switch mode power supplies. Generally, some form of current limiting protection for one or more of the secondary circuits is desired, usually the logic voltage load circuit. Thus the voltage supplying the 5 V logic circuit should not only be closely regulated to remain constant despite significant changes in load current, but also the regulator should be current limited to preclude damaging current flow in the circuitry in the event of a system or component fault.
The present invention solves both problems in a unique way. With the invention, the regulator output voltage is independently compensated for the IR drop in the regulator rectifier. Also, a current limiting circuit may be coupled to the output terminal to preclude current flow therefrom in excess of a predetermined amount.